High index glasses

ABSTRACT

A glass composition comprising: Al 2 O 3 , ZnO, and SiO 2 ; TiO 2 , in the amount of at least 10 mol % and not greater than 20 mol %; and alkaline metal oxide selected from the group consisting of MgO, CaO, SrO, BaO, or any combination thereof,
         such that the molar sum of MgO, CaO, SrO, BaO, and ZnO, in the amount in the glass composition is least 20 mol % and not greater than 35 mol %, and such that:
           the amount of BaO is 0 to 10 mol %; the amount of MgO is 0 to 10 mol %
 
the amount of CaO is 0 to 10 mol %, and the molar sum of CaO and MgO in the glass composition is less than 12.5 mol %; and
   
           rare earth metal oxides (ΣRE m O n ), in the amount of at least 1.5 mol % and not greater than 10 mol %; alkali metal oxides (ΣAlk 2 O), in the amount of greater than or equal to 0 mol % and less than or equal to 5 mol %; and not greater than 5 mol % of other components; and wherein −5 mol %≤Al 2 O 3  (mol %)−1.5 ΣRE m O n  (mol %)−ΣAlk 2 O (mol %)≤+5 mol %.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority under 35 U.S.C § 119 ofU.S. Provisional Application Ser. No. 62/773,751 filed on Nov. 30, 2018,the content of which is relied upon and incorporated herein by referencein its entirety

BACKGROUND Field

The present specification generally relates to glass compositionssuitable for use in optical displays, such as, for example, displays foraugmented reality devices or virtual reality devices, optical fibers,and optical lenses. More specifically, the present specification isdirected to high index glasses that may be used in displays foraugmented reality devices or virtual reality devices.

TECHNICAL BACKGROUND

In the recent decade, the demand for optical glasses with highrefractive index (i.e., n_(d)>1.60) has increased with the growingmarket in augmented reality and virtual reality devices. Otherrequirements for the optical glasses used in augmented reality orvirtual reality devices are good transmittance in the visible range,good glass formability, chemical durability, and relatively lowproduction cost. The manufacturing of glasses with high refractive indexis quite different from the production of display glasses that do notrequire such a high refractive index. Accordingly, the demands of highrefractive index optical glasses are not the same as the demands ofdisplay glasses, and different glass compositions may be required for 1high refractive index optical glasses than for display glasses.

Another requirement of optical glasses for use in augmented reality orvirtual reality devices is low glass density. Since many augmentedreality or virtual reality devices are made as wearable devices, theweight of the device is held by a user. Over an extended period of time,even a relatively light weight device can become cumbersome to wear.Thus, light, low-density glasses (i.e., density less than or equal to4.00 g/cm³) are desirable for use in augmented reality or virtualreality devices.

In order to reduce the cost of production, it would be preferable thatthe high index glasses have good chemical and physical properties andhave viscosity characteristics compatible with conventionalmanufacturing equipment. However, it is difficult to create a high indexglass with a combination of the desirable chemical and physicalproperties. For example, attempts to increase the refractive index ofthe glass have often caused undesirable increases in glass density thatmade the glass articles heavier. Attempts to reduce the meltingtemperature by reducing the high-temperature viscosity resulted indevitrification of the glass melt when forming glass articles, which wascaused by high liquidus temperature. Attempts to decrease the glasstransition temperature caused undesirable increases in the coefficientof thermal expansion, and attempts to increase the elastic modulus toabove 95 GPa caused undesirable raising of the glass transitiontemperature.

It is noted that some silica-free phosphate glasses may have highrefractive indexes and low CTE. These are not silicate type glasses.U.S. Pat. No. 8,691,712, discloses borate glasses having n_(d)>1.75 andα<60×10⁻⁷ K⁻¹ that comprises less than 12% silica and greater than 12%B₂O₃. However, both borate and phosphate glasses are known to have lowelastic moduli, i.e. they are not rigid enough for many applications.Also, most borate and phosphate (non-silicate) glasses comprise highamounts of harmful and/or expensive and/or heavy oxides, such as PbO,Sb₂O₃, Ta₂O₅, Gd₂O₃, Bi₂O₃, etc., which are undesirable inmass-production. Accordingly, a need exists for silicate high indexglasses that have the above-mentioned attributes, preferably have anelastic modulus above 95 GPa, and are suitable for use in an augmentedreality or virtual reality device, or for other optical components.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a linear thermal expansion coefficient (α) vs.refractive index (Nd) for several high index glasses comprising at least30 mol % SiO₂;

FIG. 2 illustrates a linear thermal expansion coefficient (α) vs.Young's elastic modulus (E) for several high index glasses comprising atleast 30 mol % SiO₂;

FIG. 3 is a schematic illustration of major effects of compositionalchanges on the glass properties of the glass embodiments disclosedherein.

SUMMARY

According to some embodiments, a glass composition comprises:

-   -   (a) Al₂O₃, ZnO, and SiO₂;    -   (b) TiO₂, in the amount of at least 10 mol % and not greater        than 20 mol %;    -   (c) alkaline metal oxide selected from the group consisting of        MgO, CaO, SrO, BaO, or any combination thereof,        -   such that the molar sum of MgO, CaO, SrO, BaO, and ZnO, in            the amount in the glass composition is least 20 mol % and            not greater than 35 mol %, and such that:            -   (i) the amount of BaO is 0 to 10 mol %;            -   (ii) the amount of MgO is 0 to 10 mol %;            -   (iii) the amount of CaO is 0 to 10 mol %, and the molar                sum of CaO and MgO in the glass composition is less than                12.5 mol %;    -   (d) rare earth metal oxides (ΣRE_(m)O_(n)), in the amount of at        least 1.5 mol % and not greater than 10 mol %;    -   (e) alkali metal oxides (ΣAlk₂O), in the amount of greater than        or equal to 0 mol % and less than or equal to 5 mol %; and    -   (f) not greater than 5 mol % of other components; and wherein        −5 mol %≤Al₂O₃ (mol^(%))−1.5 ΣRE_(m)O_(n) (mol %)−ΣAlk₂O (mol        %)≤+5 mol %.

According to some embodiments:

−3 mol %≤Al₂O₃ (mol %)—1.5 ΣRE_(m)O_(n) (mol %)−ΣAlk₂O (mol %)≤+3 mol %.

According to some embodiments, the glass composition comprises from 30mol % to 45 mol % SiO₂, and according to some embodiments from 30 to 35mol % SiO₂.

According to some embodiments, the glass composition comprises from 15mol % to 20 mol % ZnO.

According to some embodiments, the glass composition comprises less thanor equal to 15 mol % Al₂O₃. According to some embodiments, the glasscomposition comprises from 3 mol % to 15 mol % Al₂O₃, and according tosome embodiments from 10 to 15 mol % Al₂O₃. According to someembodiments, the glass composition comprises greater than or equal to 15mol % ZnO.

According to some embodiments, the glass has a refractive index n_(d)(refractive index measured at 589.3 nm) of greater than or equal to 1.66and less than or equal to about 1.83, and a density d of less than orequal to 3.9 g/cm³.

According to some embodiments. the glass composition has a meltingtemperature T_(m) that is less than or equal to 1410° C. According tosome embodiments. the glass composition has a glass transitiontemperature T_(g) that is greater than or equal to than about 600° C.and less than or equal to about 700° C. According to some embodiments.the glass composition has a crystallization onset temperature T_(x),such that the difference (T_(x)−T_(g)) is greater than or equal to about130° C.

According to some embodiments wherein the glass composition has a linearthermal expansion coefficient in the range 20-100° C., α₂₀₋₁₀₀ ofgreater than or equal to about 50×10⁻⁷ K⁻¹ and less than or equal toabout 60×10⁻⁷ K⁻¹.

According to some embodiments, the glass composition has a linearthermal expansion coefficient in the range 20-300° C., α₂₀₋₃₀₀, of lessthan or equal to 65×10⁻⁷ K⁻¹, and in some embodiments less than or equalto 60×10⁻⁷ K⁻¹.

According to some embodiments the glass composition comprises:

-   -   (a) greater than or equal to 30.0 mol % and less than or equal        to 35.0 mol % SiO₂;    -   (b) greater than or equal to 12.0 mol % and less than or equal        to 20.0 mol % TiO₂;    -   (c) greater than or equal to 10.0 mol % and less than or equal        to 15.0 mol % Al₂O₃;    -   (d) greater than or equal to 5.0 mol % and less than or equal to        10.0 mol % of earth metal oxides;    -   (e) greater than or equal to 15.0 mol % and less than or equal        to 20.0 mol % ZnO; and    -   (f) greater than or equal to 5.0 mol % and less than or equal to        15.0 mol % alkaline earth metal oxides (MgO+CaO+SrO+BaO).

According to some embodiments the glass composition comprises La₂O₃.

According to some embodiments, n_(d) is 1.78 to 1.83; and density d is3.7 g/cm³-3.9 g/cm³. According to some embodiments the glass exhibitsthe ratio (n_(d)−1)/d of greater than or equal to about 0.20 cm³/g.

According to some embodiments the glass has Young's modulus of about95-120 GPa, and linear coefficient of thermal expansion (CTE) of about60×10⁻⁷ K⁻¹ to 80×10⁻⁷ K⁻¹, within the 20° C. to 300° C. temperaturerange. According to some embodiments, the glass has Young's modulus ofabout 105-120 GPa and linear coefficient of thermal expansion α of about60×10⁻⁷ K⁻¹ to −80×10⁻⁷ K⁻¹, within the 20° C. to 300° C. temperaturerange.

According to some embodiments the glass has glass transition temperatureT_(g) of about 600° C. to about 700° C.; specific modulus of between 25and 35 GPa*cm³/gram (e.g., 28-32 GPa*cm³/gram, or 29 to 31 GPa*cm³/gram,or about 30 GPa*cm³/gram); melting temperature not greater than 1450° C.and liquidus temperature that is smaller than the melting temperature.According to some embodiments the liquidus temperature is 1300° C.,1325° C. 1350° C., 1375° C., 1400° C., 1410° C., or 1415° C.

According to some embodiments the glass has a refractive index, measuredat 589.3 nm, greater than or equal to 1.66, for example between 1.66 and1.83, or 1.78 and 1.83.

According to some embodiments the glass has a density from-greater thanor equal to 3.2 g/cm³ and less than or equal to 3.9 g/cm³.

According to some embodiments the glass has a liquidus temperature ofabout 1410° C.

According to some embodiments glass has a glass annealing temperaturefrom greater than or equal to 600° C. and less than or equal to 700° C.

According to some embodiments glass has a crystallization onsettemperature T_(x) such as the difference (T_(x)−T_(g)) is greater thanor equal to 130° C., or greater than or equal to 160° C.

According to some embodiments, glass may additionally comprise—therefractive index raising components (i.e., constituents) that areselected from a group consisting of: BaO, Nb₂O₅, MoO₃, and CeO₂.

According to some embodiments a zinc aluminosilicate glass comprises:

-   -   (a) greater than or equal to 10 weight % and less than or equal        to 20 weight % TiO₂;    -   (b) greater than or equal to 20 weight % and less than or equal        to 35 weight %    -   (MgO+CaO+SrO+BaO+ZnO), including        -   a. from 0 to 5 weight % MgO,        -   b. from 0 to 5 weight % CaO,        -   c. from 0 to 10 weight % BaO,        -   d. sum of (CaO+MgO) being less than 10 weight %;    -   (c) greater than or equal to 8 weight % and less than or equal        to 25 weight % rare earth metal oxides (ΣRE_(m)O_(n));    -   (d) greater than or equal to 0 weight % and less than or equal        to 5 weight % alkali metal oxides (ΣAlk₂O); and    -   (e) not more than 5 weight % of other species.

According to some embodiments the glass has the refractive index n_(d)about 1.78-1.83; density less than or equal to about 3.9 g/cm³; glasstransition temperature T_(g) of 600° C. to 700° C.; specific modulus ofabout 30-34 GPa*cm³/gram; and liquidus temperature of less than or equalto 1410° C., and crystallization onset temperature T_(x) greater than orequal to (T_(g)+130)° C.

According to some embodiment a glass article comprises a zincaluminosilicate glass, the glass comprising, on mole percent basis:

greater than or equal to 10 mol % and less than or equal to 20 mol %TiO₂;greater than or equal to 20 mol % and less than or equal to 35 mol %;(MgO+CaO+SrO+BaO+ZnO), including from 0 to 10 mol % MgO, from 0 to 10mol % CaO, from 0 to 10 mol % BaO, sum of (CaO+MgO) being less than 12.5mol %;greater than or equal to 1.5 mol % and less than or equal to 10 mol %rare earth metal oxides (ΣRE_(m)O_(n));greater than or equal to 0 mol % and less than or equal to 5 mol %alkali metal oxides (ΣAlk₂O); andnot more than 5 mol % of other compatible species,wherein the following is satisfied: −5 mol %≤(Al₂O₃[mol %]—1.5ΣRE_(m)O_(n) [mol %]−ΣAlk₂O [mol %])≤+5 mol %.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments described herein, including the detailed description whichfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the embodiments described herein,and together with the description serve to explain the design andadvantages of the claimed subject matter.

DETAILED DESCRIPTION Definitions

The term “annealing point,” as used herein, refers to the temperaturedetermined according to ASTM C598-93 (2013). For typical glasscompositions, the annealing point is the temperature at which theviscosity of a glass of a given glass composition is approximately10^(13.2) poise.

The term “liquidus temperature” refers to the temperature above whichthe glass composition is completely liquid with no crystallization ofconstituent components of the glass. This property is measured by thegradient method according to ASTM C829-81 Standard Practices forMeasurement of Liquidus Temperature of Glass.

The term “liquidus viscosity” refers to the viscosity of the glasscomposition at the liquidus temperature of the glass composition.

The term “α,” as used herein, refers to the mean linear coefficient ofthermal expansion (also referred to as mean coefficient of linearthermal expansion) of the glass composition over a temperature rangefrom 20° C. (room temperature (RT)) to 300° C. This property is measuredby using a horizontal dilatometer (push-rod dilatometer) in accordancewith ASTM E228-11. (I.e, unless specified otherwise α=α₂₀₋₃₀₀, linearcoefficient of thermal expansion in the temperature range of 20-300° C.)The numeric measure of α is expressed as α=

${\alpha = \frac{\Delta \; L}{L_{0}\Delta \; T}},$

whre L₀ is the linear size of a sample at some temperature within ornear the measured range, and ΔL is the change in the linear size L inthe measured temperature range ΔT. The linear thermal expansioncoefficient for the temperature range of range 20° C.-100° C. is denotedas α₂₀₋₁₀₀.

The term “refractive index”, or n_(d), refers to the refractive index ofa glass composition for the yellow d-line of sodium at about 589.3 nm,measured at room temperature, RT.

Unless specified otherwise, for the glass embodiments disclosed inherein, the refractive index was measured by using the Becke linemethod.

The density of the glass embodiments when measured, was measured at roomtemperature using Archimedes' Principle according to ASTM C693.

The elastic moduli are measured by using Resonant UltrasoundSpectroscopy, using a Quasar RUSpec 4000 available from ITW IndianaPrivate Limited, Magnaflux Division.

The glass transition temperature (T_(g)) is measured by differentialscanning calorimeter (DSC) at the heating rate of 10° K/min, after theglass was cooled in air to RT.

The exemplary embodiments of the glass(es) according to the embodimentsof the glass compositions described herein advantageously have highrefractive index, n_(d), low linear thermal expansion coefficient α attemperatures that are below the glass transition temperature T_(g), aswell as high elastic modulus E, and high specific modulus (E/d). Suchglasses can be utilized in multiple optical systems, for example asconsumer glasses, camera lenses, etc., as well as substrates forinformation recording media. In addition, the exemplary glassembodiments are light weight, and advantageously exhibit high rigidity(low deformation under external force). Other advantages of theseglasses are comparably low melting temperature and, moderate glasstransition temperature Tg (e.g., from 600° C. to 700° C.). In someembodiments the glass has a relatively high elastic modulus (E≥100 GPa),which is advantageous in media recording applications. In someembodiments the glass has elastic modulus E greater 95 GPa and less than125 GPa. And in some embodiments, glass elastic modulus E is between 98GPa and 120 GPa.

According to exemplary embodiments, the glasses have glass transitiontemperatures (T_(g)) around 600° C. making a glass compatible withconventional equipment for forming the articles; low thermal expansionto limit warping of the glass during formation—for example linearcoefficient of thermal expansion α between 50×10⁻⁷ K⁻¹ and 10×10⁻⁷ K⁻¹,preferably α<80×10⁻⁷ K⁻¹ or even α<65×10⁻⁷ K⁻¹, and making the glasscompatible with other materials like steels or alloys; low enoughmelting temperature of not greater than 1450 C, preferably not greaterthan 1400° C. (e.g. 1100° C. to 14000° C., or 1200° C. to 1400° C.melting temperature) making the glass melting process lessenergy-consuming and compatible with conventional refractories; andability not to form high stresses in the articles when cooling afterbeing formed. The last quantity mainly depends on the coefficients ofthermal expansion below and above T_(g), Young's modulus and Poisson'sratio. For example, in some embodiments the linear coefficient ofthermal expansion α is less than 100×10⁻⁷ K⁻¹, or 60-70×10⁻⁷ K⁻¹ or lessfor information recording media).

The glass embodiments described herein provide one or more of thefollowing advantages:

-   -   (α) a high refractive index n_(d) of ≥1.65 (e.g., N_(d) of 1.66;        1.75; 1.78; 1.80; 1.81; 1.83; for example, 1.67≤N_(d)≤1.83),        while having low density d (e.g., d≤3.9 g/cm³; or d≤3.5 g/cm³;        or d≤3.3 g/cm³; or d≤3.2 g/cm³, and therebetween;    -   (b) a relatively low melting temperature (1300° C.-1500° C.)        which results in lower energy consumption while making glass;        while having relatively low liquidus temperature T_(L)<1500° C.,        or (e.g., T_(L)<1450° C., T_(L)≤1410° C.;) thus avoiding or        minimizing crystallization of glass melt when forming articles;    -   (c) a relatively low glass transition temperature Tg (e.g.,        Tg≤700° C., or Tg≤650° C., or Tg≤625° C., or Tg≤600° C., or        therebetween, while having a relatively low coefficient of        thermal expansion (i.e., relatively low α value) between the        temperatures of 20° C. and 300° C. (e.g., α≤90×10⁻⁷ K⁻¹,        α≤80×10⁻⁷ K⁻¹; α≤70×10⁻⁷ K⁻¹; α≤65×10⁻⁷ K⁻¹; α≤60×10⁻⁷ K⁻¹;        which advantageously minimizes stresses and warping when cooling        the glass articles after being formed; and/or    -   (d) high refractive index (n_(d)>1.75; or n_(d)>1.78, or        n_(d)>1.81) while having low linear thermal expansion        coefficient (α<80×10⁻⁷ K⁻¹, or α<70×10⁻⁷ K⁻¹, or α<60×10⁻⁷ K⁻¹).

For example, in some embodiments the glass density d is 3.2 g/cm³≤d≤3.9g/cm³; or 3.5 g/cm³≤d≤3.95 g/cm³. In some embodiments the glasstransition temperature is 575° C.≤Tg≤700° C.; or 600° C.≤Tg≤700° C. Insome embodiments the glass density d is 3.2 g/cm³≤d≤3.9 g/cm³ (e.g., 3.5g/cm³≤d≤3.95 g/cm³) and the glass transition temperature is 575°C.≤Tg≤700° C.; (e.g., 600° C.≤Tg≤700° C. In some embodiments, 55×10⁻⁷K⁻¹≤α≤90×10⁻⁷ K⁻¹; or 55×10⁻⁷ K⁻¹≤α≤80⁻⁷ K⁻¹; or 55×10⁻⁷ K⁻¹≤α≤65⁻⁷ K⁻¹;or 55×10⁻⁷ K⁻¹≤α≤60×10⁻⁷ K⁻¹. In some embodiments, n_(d)>1.75 andα<80×10⁻⁷ K⁻¹. In some embodiments n_(d)>1.78 and α<70×10⁻⁷ K⁻¹, and insome embodiments n_(d)>1.79 and α<60×10⁻⁷ K⁻¹.

In at least some embodiments the elastic modulus is above 100 GPa, orabove 110 GPa, or even above 115 GPa. This high value prevents the glassarticles and the devices comprising these articles, such as lenses,displays or information storage media, from deformation under loading.

High elastic modulus E together with relatively low density d helps toenable high resistance of the light-weight glass articles to externalmechanical forces, which is especially important for informationrecording media. The mentioned property can be quantitatively describedby using such a characteristic as specific modulus (E/d), which has highvalue, for example 30 GPa·cm³/gram.

High elastic modulus is also advantageous for the glasses utilized invirtual reality or augmented reality systems, because high elasticmodulus of the glass helps to prevent (or minimizes) deformation of theelements of the electric circuit that creates the image in thesesystems.

At the same time, however, it is known that high elastic modulus E inthe glass may cause high thermal stresses within the glass when it isbeing cooled during the manufacturing process. High stresses within theglass are undesirable when the glass is used in optical or in consumerapplications (such as the devices for virtual reality or augmentedreality, or high-index lenses).

The likelihood of occurrence of high thermal stresses in opticalarticles can be evaluated using a ratio E·α/(1−ν), where E representsYoung's modulus, where a is the mean coefficient of linear thermalexpansion and ν is Poisson's ratio. Accordingly, the lower the ratioE·α/(1−ν), the less the amount of stress. Thus, it is preferable thatthe glass embodiments described herein have the ratio E·α/(1−ν) of about1.0 MPa·K⁻¹ or less.

In order to maintain the ratio E·α/(1−ν) at the desired low enough level(≤1.0 MPa·K⁻¹ or less) while simultaneously having a high elasticmodulus E, the glass should have low value of CTE (i.e., low α).However, it was known that it is difficult or impossible to make a glasswith high refractive index n_(d) and low CTE (low α), especially if aglass comprises some significant amount of silica (e.g. greater than orequal to 30 mol % SiO₂) and does not have large amounts of ecologicallyharmful (undesirable) species, such as lead, antimony or tantalum.

Without these undesirable species, it was only possible to have glasscompositions that have either not a very high refractive index(n_(d)<1.75), or that have high CTE (high α, where α>65×10⁻⁷ K⁻¹ in thetemperature range of 20−300° C.). This is illustrated in FIG. 1 whichshows available data for these two properties for comparative glasses(marked as glasses 1-7 in FIG. 1) that comprise greater than or equal to30.0 mol % SiO₂ and that are essentially free of such undesirableelements as Fe (colorant), Pb, Sb and Ta. The comparative glasses shownin FIG. 1 have refractive index n_(d)>1.75, but also have α valuesbetween about 67×10⁻⁷ K⁻¹ and 80×10⁻⁷ K⁻¹, in the temperature range of20° C.-300° C. FIG. 1 also illustrates measured data for some of theglass embodiments described herein (marked by a star symbol and as glass8, see for example the gray colored area of FIG. 1). All glassesdepicted in FIG. 1 have n_(d)≥1.75 and α≤80×10⁻⁷ K⁻¹.

As it is shown in FIG. 1, the exemplary embodiments of the glasscompositions disclosed herein exhibit low CTE values (α<70×10⁻⁷ K⁻¹,more preferably α<68×10⁻⁷ K⁻¹, preferably α<65×10⁻⁷ K⁻¹, or evenα<60×10⁻⁷ K⁻¹) while having high refractive indices n_(d), exceeding1.75 or even 1.79. At least some exemplary embodiments of the glasscompositions disclosed herein exhibit low CTE values (α<70×10⁻⁷ K⁻¹,more preferably α<68×10⁻⁷ K⁻¹, preferably α<⁶⁵×10⁻⁷ K⁻¹, or evenα<60×10⁻⁷ K⁻¹) while having high refractive indices n_(d), for examplebetween 1.75 and 1.8.

It is especially difficult to design a glass composition with high indexand low CTE if the glass is required to have a high Young's elasticmodulus. This difficulty is illustrated in FIG. 2, where we showavailable data for the Young's modulus and CTE values for a set ofcomparative silicate glasses (indicated by symbols *, +, and ▪ in FIG.2) that comprise greater than or equal to 30.0 mol % SiO₂ (without othercompositional limitations) and that have a refractive index n_(d)>1.75and Young's modulus E≥90 GPa. FIG. 2 also illustrates measured data forsome of the manufactured high refractive index glass embodimentsdescribed herein (marked by star symbols in FIG. 2). The glassembodiments shown in FIG. 2 have n_(d)≥1.75 and α≤80×10⁻⁷ K⁻¹.

As is clear from the figure, the exemplary embodiments of high-indexsilicate glass compositions disclosed herein exhibit low CTE values(α<80×10⁻⁷ K⁻¹, α<65×10⁻⁷ K⁻¹, or even α <60×10⁻⁷ K⁻¹) at young'smodulus E>100 GPa, or E>110 GPa, or even E>115 GPa. The uniquecombination of very high elastic modulus (E>115 GPa) and low CTE(α<60×10⁻⁷ K⁻¹) for glasses with high refractive index (n_(d)>1.78, oreven n_(d)>1.79) makes it possible to keep the high-index material veryrigid and prevent high thermal stresses in it during manufacturing.

The glass embodiments shown in FIG. 2 are free of the environmentallyharmful, expensive, and/or heavy species (such as the following oxidesPbO, Sb₂O₃, Ta₂O₅, Gd₂O₃, Bi₂O₃, etc.) which are undesirable for use inmass-production of glass. All glass embodiments described hereincomprise silica as a main network former, which makes them lessexpensive to manufacture.

The embodiments of the glasses described herein comprise three majorconstituents (also referred as components herein): silica (SiO₂),titania (TiO₂) and zinc oxide (ZnO). The glasses may also comprisealkaline earth metal oxides and rare earth metal oxides. The glasses mayalso optionally comprise alkali metal oxides and other compatibleconstituents in low concentrations (e.g., 5 mol %, or less).

The glass embodiments described herein can be utilized, for example, inat least two kinds of applications: as optical glasses with highrefractive indices (below, we use the term “high-index glasses”) and/oras substrates for information recording media. The high-index glassesmay be used for various applications, such as displays for virtualreality and augmented reality, high-index lenses, lasers, etc.

FIG. 3 schematically illustrates major effects of changing the contentof the amounts of above-mentioned major glass components (SiO₂, ZnO andTiO₂) on the glass properties of the embodiments described herein. Itshould be understood that the present disclosure is directed to glasscompositions having multiple potential applications, where differentattributes may be relatively more or less important, depending on thedesired use or application for a particular glass. Therefore, dependingon a particular use or application, different amounts of components(i.e., constituents) may be preferable for different glass embodimentsdescribed herein, in order to provide different desired glass propertiesfor different glass applications discussed herein.

In embodiments of glass compositions described herein, theconcentrations of components (e.g., SiO₂, ZnO, TiO₂, Al₂O₃, and thelike) are given in mole percent (mol %) on an oxide basis, unlessotherwise specified. Components of the glass composition according toembodiments are discussed individually below. It should be understoodthat any of the variously recited ranges of one component may beindividually combined with any of the variously recited ranges for anyother component.

As described above, the glass compositions disclosed herein comprisesilica, SiO₂, as a glass network former. Silica increases the viscosityof glasses through the whole temperature range, increasing liquidusviscosity, which enables the glass melt to avoid crystallization attemperature ranges near the liquidus temperature. Also, adding moresilica to a glass composition causes a reduction in glass density andCTE, which is desirable. However, silica significantly reduces therefractive index of glass. Adding too much silica to the glasscomposition also increases the melting temperature of the glass, whichmay not desirable. Applicants discovered that if the content of SiO₂ ina glass composition is less than about 30 mol %, the silica based glassis difficult to form. If the content of SiO₂ in a glass composition isgreater than about 45 mol %, the refractive index of the glass becomestoo low.

Accordingly, for the embodiments of high-index glasses disclosed herein,the content of silica in the glass composition is preferably within therange from about 30 to about 45 mol %. In some embodiments, the glassescomprise SiO₂ in the amount from about 30 to about 33 mol %. Theseembodiments have the highest refractive index and lowest CTE, but theirglass forming ability is not as good as it is for glasses with higheramount of silica. In some other embodiments, the glasses comprise SiO₂in the amount from about 40 to about 45 mol %; these glasses arecharacterized by better glass forming ability, but lower refractiveindex and higher CTE, which may be preferable for such applications asinformation storage devices. Intermediate SiO₂ concentration ranges,such as 33 to 40 mol %, 33 to 35 mol %, or 35 to 40 mol %, may bepreferable for the applications where these properties are equallyimportant, for example the glass composition for use in makinghigh-index lenses. Thus, it should be understood that the content ofsilica can be changed within the 30-45 mol % range, depending on thepreferable combination of properties, i.e. the comparative importance ofthe glass forming ability, refractive index and CTE, which depends onthe particular application of glasses as described above.

The glass composition also comprises zinc oxide, ZnO. Zinc oxideimproves mechanical characteristics of the glass and increases Young'smodulus of the glass, while not significantly increasing the density ofglass or CTE. Because zinc oxide increases the refractive index of theglass (relative to silica), while not appreciably increasing the densityd, the addition of zinc oxide to silica results in an increase in theratio (n_(d)−1)/d, which is advantageous in high index glasses. Also,zinc oxide can be used to stabilize the high-index species, such asTiO₂, ZrO₂, Nb₂O₅, etc., as far as it chemically reacts with thesespecies and, therefore, accommodates them to the glass structure (seeFIG. 3). However, if the glass includes alumina, when the content ofzinc oxide becomes too high zinc oxide may react with the excess ofalumina in the glass, forming a mineral gahnite (ZnAl₂O₄) that mayprecipitate from a glass melt at high temperatures. In order toneutralize this effect, it is desirable to provide additional componentsin the glass, such as alkali and alkaline earth metal oxides, and/orrare earth metal oxides as described below.

Accordingly, the preferable range of ZnO in the embodiments of the glasscompositions is mostly governed by the amounts of other high-indexspecies added to the glasses. For the glasses with highest refractiveindexes, as well as for glasses with relatively low (but still highenough) Young's modulus, like 100 GPa or less, higher concentrations ofZnO, such as about 20 mol % or more, are preferable. In the case whenthe requirements to the refractive index are not as high, but highrigidity is more important, the content of ZnO may be lower, such asabout 15 mol % or less. In the case when these properties are equallyimportant, the intermediate content of ZnO, such as 15-17 mol %, or17-18 mol %, or 18-20 mol %, may become more preferable.

As described above, the third major component of the glass embodimentsis titania, TiO₂. Titania greatly increases refractive index in a glassand has a comparably low impact to the density. Furthermore, addingtitania to a glass increases the elastic modulus and fracture toughnessof the glass and reduces the CTE. Thus, the addition of titaniaadvantageously may bring very desirable glass characteristics (see FIG.3). However, when added to peraluminous glasses (characterized bypositive values (in mol %), of the quantity (Al₂O₃-ΣR₂O)), titania mayprecipitate from the melt in the form of rutile or other minerals. Inturn, precipitation of titania raises the liquidus temperature of theglass, which may not be desirable. In addition, in high concentrations,titania may bring some coloring to a glass, which may not be desirablefor some optical glass applications. Accordingly, the content of titaniain the glasses should be as high as possible (e.g., 10 to 20 mol %) toreach better properties for the refractive index, density, CTE andelastic modulus, but its maximum amount in the glass composition is alsolimited by the above-described negative effects, such as devitrificationof the melt and/or coloring the glass.

According to some embodiments, the glass compositions compriserelatively small amounts of TiO₂, i.e. from about 10 to about 13 mol %TiO₂. These glasses do not exhibit the tendency of devitrification, butprovide moderately high refractive index, higher CTE and lower Young'smodulus. These glasses may be preferable, for example, for use in thelenses where the highest refractive index is not demanded.

According to some embodiments, the glass compositions comprise fromabout 13 to about 15 mol %, or from about 15 to about 18 mol % of TiO₂.These glasses provide comparably high refractive index (the higher thecontent of TiO₂, the higher is the refractive index), and do notdevitrify when being fast quenched, e.g. cooled between two metalplates.

According to some other embodiments, the glass compositions comprisefrom about 18 to about 20 mol % TiO₂. These glasses exhibit very highrefractive index (up to n_(d)>1.81), but may also exhibit tendencytoward devitrification and coloring.

Accordingly, the preferable content of TiO₂ in the glass compositionsmay be from about 10 to about 20 mol %, with variations depending on aparticular application, i.e. comparable importance of mechanical,optical and crystallization properties, such as, for example, from 10 to12 mol %, from 12 to 14 mol %, or from 14 to 16 mol %, or from 16 to 18mol %, or from 18 to 20 mol %.

Then, the glass composition may optionally comprise rare earth metaloxides (also referred to herein as “rare earths”). For colorlessglasses, the rare earth metal oxides may include Ce₂O₃, Pr₂O₃, Nd₂O₃,Sm₂O₃, Eu₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, or combinationsthereof. If no color is required or desired in the glass, then a glasscomposition may include lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃),gadolinium oxide (Gd₂O₃), ytterbium oxide (Yb₂O₃), lutetium oxide(Lu₂O₃), or combinations thereof. Furthermore, the glass composition mayalso contain small amounts of these rare earth metal oxides when thepresence of some coloring is acceptable, such as in thin lenses forconsumer glasses, or sunglasses with required optical performance. Rareearth metal oxides, when added to a glass composition, increase therefractive index of the glass, which improves optical performance of theglass. Also, rare earth metal oxides may reduce the liquidus temperatureof the glass, preventing the glass melt from divertifying. The rareearth metal oxides improve mechanical properties of the glass, andprovide, for example a high elastic modulus, which is one of therequirements for information recording media and is also a desirablefactor for optical glasses.

However, rare earth metal oxides may also increase the glass density,which may somewhat decrease the ratio of (n_(d)−1)/d. For that reason,the use of the lightest (and the least expansive) of rare earth metaloxides, La₂O₃ and Y₂O₃, is preferable. In the case when the low glassdensity is relatively more important, the lighter of the rare earthmetal oxides, Y₂O₃, may be preferable. Otherwise, it may be preferableto use La₂O₃ as the least expensive of rare earth metal oxides. Theimpacts of the other rare earth oxides on other properties disclosedherein are similar. Accordingly, other rare earth oxides may also beused in the glass compositions; however, in terms of the combination ofproperties described herein, they do not provide significant advantagesover La₂O₃ and Y₂O₃.

In the disclosed exemplary embodiments, we used the least expensive ofthe rare earth metal oxides (La₂O₃) to demonstrate that the desiredcombination of multiple properties of high-index glasses can be achievedfor a relatively low cost. As described above, La₂O₃ may be added to theoptical glass composition to increase the refractive index of theoptical glass. However, when too much La₂O₃ is added into glasscomposition, the density of the glass increases, and the glass melts maydevitrify upon cooling.

Preferably, the total amount of rare earth metal oxides in the glasscomposition is 1.5 mol % to 10 mol %. For example, the addition of therare earth metal oxides in relatively high concentrations, such as, forexample, 6 mol % or more (e.g., 6 to 8 mol %, or 8 to 10 mol %) may bepreferable when very high values of the refractive index together withthe highest mechanical performance (such as, the highest Young'smodulus) are desired. When the highest refractive index is not demanded,and/or the requirements to the mechanical performance are not as high,the rare earth metal oxides may be used in lower concentrations (suchas, for example, from 4 to 6 mol %, or from 2 to 4 mol %), or even insmaller concentrations (such as 1-2 mol %), or even not used at all.

The glass compositions may comprise the alkaline earth metal oxides, forexample be BeO, MgO, CaO, SrO, BaO or combinations thereof. The alkalineearth metal oxides (below, we also use the term “alkaline earths”) mayneutralize some excess of alumina, keeping the liquidus temperature inthe acceptable range, say, not more than 1350-1450° C.

However, beryllium oxide (BeO), the lightest of alkaline earths, worksless effectively and, in addition, reduces the refractive index of theglass; therefore, BeO is not a preferred component in these glasses.

Magnesia, MgO, is the lightest of the alkaline earth metal oxides(except for ecologically undesirable BeO) and makes the greatest impactin the mechanical performance, which exhibits as increasing the Young'smodulus and other elastic moduli (also, it is known to increase thefracture toughness). In addition, MgO (relative to other alkaline earthmetal oxides) has the lowest impact on CTE. However, it also has thelowest impact (relative to other alkaline earth metal oxides) on therefractive index of glass. Accordingly, a relatively high content ofMgO, such as 6-8 mol % or even more, up to 10 mol %, may be beneficialfor applications that do not require the highest values of therefractive index, but where low CTE and/or high mechanical performanceare demanded. In other cases, MgO may be used in small concentrations(such as 2-4 mol %, or 4-6 mol %), or not used at all. In general, thepreferable content of MgO may vary from 0 to about 10 mol %.

Calcium oxide, CaO, basically works similarly to MgO, but has a somewhatgreater impact on the refractive index and CTE and a little bit lowerimpact on mechanical properties, such as Young's modulus. However, itworks significantly better than MgO as a species that stabilizes thehigh-index components, such as ZrO₂ or Nb₂O₅. Also, unlike magnesia, CaOdoes not form very refractory minerals in the excess of alumina, whichprevents the peraluminous glasses from devitrification. Accordingly, theuse of CaO is preferable in the cases when light weight very highrefractive index is required, but the rigidity (Young's modulus) is notrequired to reach the highest possible values, and/or the lowest CTE isnot demanded. In these cases, the glass compositions may comprise 6-8mol % CaO or even up to 10 mol % CaO. In the cases when the requirementsto such properties as rigidity and CTE are stronger, but very highrefractive index is still demanded, the content of CaO in glasscompositions may be somewhat less, such as 4-6 mol %, or 2-4 mol %.Otherwise, e.g. when the mechanical performance is the most importantand the highest refractive index not as much, the amount of CaO in theglass compositions may be rather small (such as 2-4 mol %), or a glassmay even not comprise CaO at all. In general, the preferable content ofCaO may vary from 0 to about 10 mol %.

Barium oxide, BaO, works somewhat differently from CaO and MgO. The mostimportant thing for BaO is that it not only has the highest impact tothe refractive index, but, in addition, works effectively as a speciesthat stabilizes other high-index species, such as TiO₂, ZrO₂ etc. As aresult, addition of BaO in glass, considering also the possibility ofincreasing the amount of other high-index species, generates the highestimpact on the refractive index. However, among the above alkaline earthmaterials, BaO has the highest impact on CTE, lowest impact on theYoung's modulus, and highest impact on density. Accordingly, addition ofBaO is beneficial in the case when the highest values of the refractiveindex are demanded. In this case, the glass composition may comprisemore than 5 mol % BaO, or even more, up to 10 mol %. However, in thecase when the properties described herein are demanded in combination,the highest content of BaO is not beneficial, and the preferable valuesare intermediate, like 4-6 mol %. In the case when the highestrefractive index is not demanded, the glass composition may compriserather small amount of BaO, such as 1-2 mol % or 2-4 mol %, or notcomprise this component at all. In general, the preferable content ofBaO may vary from 0 to about 10 mol %.

Strontium oxide, SrO, works intermediately between CaO and BaO, and inthe glasses considered herein, it does not bring specific advantages,comparing to CaO, BaO, or their combination. However, technologically itis beneficial to use a single component rather than the mix of two. Fromthis viewpoint, the use of SrO may become beneficial as a replacementfor a mixture (CaO+BaO) in the cases when, otherwise, CaO and BaO wouldbe desired in the proportions close to 1:1. Also, in some compositionalspaces, the use of several different alkaline earths may reduce theliquidus temperature, comparing to the use of a single species; in thiscase, SrO may also be useful components. In the above-mentioned cases,the preferable amount of SrO in glasses is similar to those for CaO andMgO, i.e. it may vary from 0 to about 10 mol %, depending on theproperty requirements.

In addition to the above components, the optical glass may includealkali metal oxides, i.e. Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O. The alkali metaloxides may be added to modify various properties of the glasscomposition, such as, for example, melting temperature, viscosity,mechanical strength, chemical durability, CTE and refractive index. Thealkali metal oxides (also referred to herein as “alkalis”) are the veryeffective suppressors for precipitation of alumina in forms of mulliteand/or corundum. This happens because the alkalis in the glass meltreact with alumina, forming aluminosilicates that at small enoughconcentrations are not refractory and, correspondingly, do notprecipitate from the melt, preventing formation of otheralumina-containing species, such as mentioned mullite and corundum.Similarly, they work efficiently as suppressors of crystallization ofother refractory species, including the high-index species, such asTiO₂, ZrO₂, Nb₂O₅, etc. Accordingly, the addition of alkalis to a glasscomposition mostly reduces the liquidus temperature, that is beneficial.However, the alkali oxides reduce both refractive index and Young'smodulus, and raise the CTE, which is undesirable.

Among the alkali metal oxides, the use lithium oxide (Li₂O) is mostlymore preferable than the use of other alkali metal oxides, because itprovides less negative impact on refractive index and on Young'smodulus, and does not raise the CTE that other alkali metal oxides do.However, in the cases when the highest effect on the liquidustemperature is required, the glasses may comprise Na₂O, K₂O or otheralkalis.

Thus, in order to reduce the liquidus temperature of the glass to adesirable level while minimizing the negative effects on refractiveindex, Young's modulus, and CTE, the alkali metal oxides may optionallybe added to a glass in limited amounts (e.g., up to 5 mol %), or notadded at all if the liquidus temperature is not required to be reduced.When using the greatest amounts of alkalis, such as greater than 3 mol%, the use of Li₂O is beneficial, in order to minimize the negativeeffects on the refractive index, mechanical properties and CTE.Otherwise, other alkalis may also be used. Accordingly, the preferablecontent of alkali oxides in the glass compositions depends on the needto reduce the liquidus temperature and may vary between 0 and about 5mol %, such as 0-1 mol %, or 1-2 mol %, or 2-3 mol %, or 3-4 mol %, or4-5 mol %.

Refractive index raising components, in addition to TiO₂, ZnO and rareearth metal oxides discussed above, may also include ZrO₂, MoO₃, WO₃,etc. They may be used in small amounts, such as 0 to 5 mol %. However,it should be noted that the addition of zirconia (ZrO₂) to peraluminousglasses may raise the liquidus temperature up to 1400-1600° C. and evenmore, which may cause devitrification of the melt. Other species thatraise the refractive index are either not ecological (e.g. Ta₂O₅, Sb₂O₃,etc.), or considerably more expensive than the species described above,or both. Accordingly, these species may be used in the gasses in somecases, but the use of these components may not be efficient in terms ofcost-performance ratio.

Other types of components that may be utilized used in the glasscompositions are finer agents. Finer agents are used to remove gasbubbles from a glass melt and make it more homogeneous. For thispurpose, different species of finer agents may be used, but the mostpreferable among them are those that also make desirable impact in theabove-described properties, most of all, the refractive index. For thispurpose, the glasses may comprise such species as CeO₂, SnO₂, etc., inthe amounts required for that; mostly, this is from 0 to about 1 mol %,unless a greater amount is needed for other purposes. For example,cerium oxide, CeO₂, as one of the rare earth metal oxides (see above),may also reduce the liquidus temperature in some glass compositions.

Also, the glass composition of the embodiments described herein mayoptionally comprise small amounts of other compatible species orcomponents, such as, for example, additional network formers, e.g. boronoxide (B₂O₃), phosphorus oxide (P₂O₅), etc. In particular, B₂O₃ and P₂O₅are known to efficiently reduce the CTE of glass and improve the glassforming ability. However, these species also reduce the refractive indexand Young's modulus, which is very undesirable. Therefore, these speciesmay be used in small amounts only in the case when there is no other wayto achieve the effects that they impact. In other cases, their use isnot desirable.

Finally, as stated above, the glass compositions may comprise differentcomponents in various combinations. For this reason, it is alsoimportant to understand that different components may chemicallyinteract with each other, and these interactions, sometimes rathercomplicated (such as interaction of three, four or even more componentstogether), may also affect some of the glass properties. Theabove-mentioned rare earth metal oxides (RE_(m)O_(n)) are known to reactwith multiple components, especially concerning the alumina (Al₂O₃) andalkali metal oxides (Alk₂O), which, in turn, also react with alumina.Therefore, there are some desirable proportions that, being satisfied,optimize the properties of glass. So, as we mentioned above, alumina,being contained in a glass composition in high concentrations, may raisethe liquidus temperature of the glass and, accordingly reduce its theliquidus viscosity, which is not desirable. We also mentioned that thiseffect can be minimized or compensated for by adding rare earths metaloxides and/or alkali metal oxides a glass composition. However, as wefound, this effect is observed when these three kinds of components,Al₂O₃, RE_(m)O_(n) and Alk₂O, are taken in the proportions that satisfythe following ratio: P=[Al₂O₃ (mol %)−ΣR₂O (mol %)−1.5 ΣRE_(m)O_(n) (mol%)]≈0, where ΣR₂O is the total content of alkali metal oxides andΣRE_(m)O_(n) is the total content of rare earth metal oxides. Inpractice, however, exact zero value is not required, but, rather, it isdesired that the mentioned quantity would not be big, such as −3 mol%≤P≤+3 mol %, or −5 mol %≤P≤+5 mol %. In some cases, it may be possibleto reach acceptable glass characteristics without satisfying theseproportions; however, when |P|≤5 mol %, the glass compositionspreferably have better overall combinations of properties.

As disclosed above, the density of the optical glass may, in one or moreembodiments, be relatively low. In at least some embodiments, thedensity was measured (according to ASTM C693). The density d of theoptical glass described herein is less than or equal to 4.00 g/cm³, suchas less than or equal to 3.9 g/cm³, less than or equal to 3.5 g/cm³, orless than or equal to 3.2 g/cm³. In one or more embodiments, the densityof the optical glass may be from greater than or equal to 3.2 g/cm³ toless than or equal to 4.00 g/cm³, such as from greater than or equal to3.25 g/cm³ to 4.00 g/cm³, from greater than or equal to 3.4 g/cm³ toless than 4.00 g/cm³, or from greater than or equal to 3.5 g/cm³ to lessthan 4.00 g/cm³.

Liquidus temperature as used herein is measured by the gradient furnacemethod. This method conforms to ASTM C829-81 Standard Practices forMeasurement of Liquidus Temperature of Glass.

As described herein “Young's modulus” is measured by Resonant UltrasoundSpectroscopy, using a Quasar RUSpec 4000 manufactured by Magnaflux.According to the exemplary embodiments the Young's modulus of theoptical glass is greater than or equal to 100 GPa. For example, in someembodiments, the Young's modulus of the optical glass is greater than orequal to 100 GPa and less than or equal to 120.0 GPa, such as greaterthan or equal to 105 GPa and less than or equal to 120 GPa, greater thanor equal to 85.0 GPa and less than or equal to 100.0 GPa, or greaterthan or equal to 90.0 GPa and less than or equal to 95.0 GPa, and allranges and sub-ranges between the foregoing values.

The thermal stability of the optical glass composition can be determinedby measuring the difference between T_(x) and T_(g) (i.e., T_(x)-T_(g)).The T_(x)-T_(g) value is measured as described hereinabove. In one ormore embodiments, the T_(x)−T_(g) of the optical glass may be fromgreater than or equal to 100° C. to less than or equal to 250 C, such asfrom greater than or equal to 130° C. to less than or equal to 170° C.

In some embodiments, the coefficient of thermal expansion (α) of theglass composition may be between about 50×10⁻⁷ K⁻¹ and 65×10⁻⁷ K⁻¹ andall ranges and sub-ranges between the foregoing values. The coefficientof thermal expansion (α) is determined by using a push-rod dilatometerin accordance with ASTM E228-11.

As disclosed above, optical glasses according to embodiments disclosedand described herein may be used in augmented reality devices, virtualreality devices, or information recording media.

EXAMPLES

Embodiments will be further clarified by the following examples.

Representative glass compositions and properties are summarized inTables 1A and 1B, respectively. Table 1B lists disclosed examples ofglass compositions.

One or more of glass compositions having components listed in Tables 1Aand/or 1B below were prepared by conventional glass forming methods.These glasses are made from batches (e.g., glass melts of 1000 g 100%theoretical yield; typical yields were about 900 g or 90 wt % due to,e.g., mechanical loss) of source or starting materials including—forexample, B₂O₃, Al₂O₃, SiO₂, CO₃, Na₂CO₃, CaCO₃, BaCO₃, ZnO, ZrO₂, TiO₂,La₂O₃, Nb₂O₅, SnO₂ and other common species that are melted in Ptcrucibles at from 1350° C. to 1500° C. in air with an aluminum cover.More specifically, the constituents of the glass composition(s) weremelted in platinum crucibles between 1500° C. and 1600° C. for 5 to 6hours. The glasses were then cooled between two steel plates andobtained samples of few mm thickness annealed for 1-5 hours near theanneal temperatures given in Table 2. Multiple samples of each glasscomposition were prepared. Each of the glass samples were tested for therefractive index n_(d) and density. Some of the example compositionswere also tested glass transition temperature (T_(g)), coefficients ofthermal expansion (α) below and above T_(g), viscosity, Young's modulus,Poisson's ratio, and liquidus temperature.

In Table 1A, all glass components are provided in mol %. (Some of theglass compositions having constituents (also referred to as componentsherein) listed in Table 1A below were modeled. These are denoted by anasterisk (*). The glass properties of these compositions were modeledand are similar to those that are measured from the prepared glasssamples).

Various properties of the glasses formed according to Tables 1A and 1Bare provided below in Table 2.

TABLE 1A Compositions of exemplary glasses in mol % Mole % by batch Ex.1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12Ex. 13 SiO2 43.66 31.85 32.23 32.41 30.07 43.99 42.03 42.85 42.34 41.9941.65 41.54 40.93 TiO2 10.34 15.54 19.39 15.00 14.58 15.60 11.71 14.4515.93 14.40 17.01 18.52 17.80 Al2O3 4.65 12.65 10.63 12.22 12.19 1.000.84 0.93 0.96 0.91 0.96 1.00 0.96 La2O3 1.85 5.98 6.82 5.72 6.32 1.730.35 0.98 0.75 0.46 0.39 0.40 0.00 Y2O3 0.00 0.00 0.00 0.00 0.00 5.557.88 6.87 7.40 7.84 8.13 8.20 8.91 CeO2 0.01 0.04 0.04 0.03 0.04 0.010.01 0.01 0.01 0.01 0.01 0.01 0.01 ZnO 19.36 17.34 19.90 17.82 17.7313.49 10.88 11.39 9.42 9.39 7.25 6.21 5.14 Li2O 0.97 0.22 4.25 0.31 0.601.10 2.14 2.28 3.83 3.59 5.45 6.39 6.97 Na2O 3.38 0.83 0.07 0.07 0.080.00 2.96 1.20 0.78 1.71 0.66 0.00 0.68 K2O 0.29 0.00 0.00 0.00 0.703.88 1.62 2.54 1.90 1.52 1.06 0.90 0.19 MgO 5.91 0.60 0.02 1.76 2.060.41 7.82 3.69 3.29 5.39 3.60 2.40 4.23 CaO 6.33 7.79 3.37 7.61 7.982.27 5.87 3.92 3.84 4.82 4.10 3.60 4.51 SrO 0.00 0.14 0.05 0.13 0.145.45 2.29 3.80 3.39 2.69 2.69 2.80 1.91 BaO 0.00 4.64 1.92 4.53 4.933.34 1.35 2.78 3.59 2.79 4.19 5.00 4.65 Nb2O5 0.30 0.00 0.49 0.00 0.000.86 0.42 0.59 0.44 0.37 0.25 0.20 0.06 SnO2 0.30 0.30 0.30 0.30 0.300.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 B2O3 0.78 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO2 0.62 2.08 0.49 2.08 2.281.32 1.82 1.74 2.16 2.15 2.61 2.84 3.05 MoO3 1.25 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Alk2O 4.64 1.05 4.33 0.38 1.384.98 6.72 6.02 6.51 6.82 7.17 7.29 7.84 REmOn 1.86 6.01 6.86 5.75 6.367.29 8.24 7.86 8.16 8.31 8.53 8.61 8.92 MgO + CaO 12.25 8.39 3.40 9.3710.04 2.68 13.69 7.61 7.13 10.21 7.70 6.00 8.74 MgO + CaO + 8.02 12.695.28 13.06 13.78 13.69 13.36 13.88 14.58 14.27 15.25 15.76 15.87 SrO +BaO MgO + CaO + 31.61 30.52 25.27 31.85 32.83 24.96 28.21 25.58 23.5325.08 21.83 20.01 20.44 SrO + BaO + ZnO Al2O3 − −2.78 2.58 −3.99 3.211.28 −14.92 −18.24 −16.88 −17.79 −18.38 −19.01 −19.21 −20.26 1.5REmOn −Alk2O

TABLE 1B Compositions of exemplary glasses in weight % Weight % by batchEx. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex.12 Ex. 13 SiO2 35.46 20.45 20.88 21.10 19.15 29.64 30.98 30.06 29.6830.16 29.44 29.02 29.26 TiO2 11.16 13.27 16.70 12.98 12.34 13.97 11.4713.48 14.84 13.75 15.98 17.20 16.92 Al2O3 6.41 13.79 11.69 13.50 13.171.14 1.05 1.11 1.14 1.11 1.15 1.19 1.17 La2O3 8.14 20.81 23.95 20.1821.81 6.32 1.40 3.73 2.85 1.79 1.50 1.52 0.00 Y2O3 0.00 0.00 0.00 0.000.00 14.05 21.83 18.11 19.49 21.16 21.59 21.53 23.94 CeO2 0.03 0.07 0.080.06 0.07 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 ZnO 21.30 15.08 17.4615.71 15.29 12.31 10.86 10.82 8.94 9.14 6.94 5.88 4.98 Li2O 0.39 0.071.37 0.10 0.19 0.37 0.78 0.80 1.34 1.28 1.92 2.22 2.48 Na2O 2.83 0.550.05 0.05 0.05 0.00 2.25 0.87 0.56 1.27 0.48 0.00 0.50 K2O 0.37 0.000.00 0.00 0.70 4.10 1.87 2.79 2.09 1.71 1.17 0.99 0.21 MgO 3.22 0.260.01 0.77 0.88 0.19 3.87 1.74 1.55 2.60 1.71 1.13 2.03 CaO 4.80 4.672.04 4.62 4.74 1.43 4.04 2.57 2.51 3.23 2.70 2.35 3.01 SrO 0.00 0.160.06 0.15 0.15 6.33 2.91 4.60 4.10 3.33 3.28 3.37 2.36 BaO 0.00 7.603.17 7.52 8.00 5.74 2.54 4.98 6.42 5.11 7.56 8.91 8.48 Nb2O5 1.08 0.001.40 0.00 0.00 2.56 1.37 1.83 1.36 1.18 0.78 0.62 0.19 SnO2 0.61 0.480.49 0.49 0.48 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 B2O3 0.73 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO2 1.03 2.740.65 2.77 2.97 1.82 2.75 2.50 3.11 3.17 3.78 4.07 4.47 MoO3 2.43 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Alk2O 3.59 0.621.42 0.15 0.94 4.47 4.91 4.46 3.99 4.26 3.57 3.21 3.19 REmOn 8.17 20.8824.03 20.24 21.88 20.39 23.25 21.86 22.36 22.97 23.11 23.07 23.96 MgO +CaO 8.02 4.93 2.05 5.39 5.62 1.61 7.91 4.30 4.06 5.83 4.41 3.47 5.04MgO + CaO + SrO + 29.32 27.77 22.74 28.77 29.07 26.00 24.22 24.70 23.5223.41 22.19 21.64 20.85 BaO + ZnO

TABLE 2 Properties of exemplary glasses* Property Ex. 1 Ex. 2 Ex. 3 Ex.4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Density d (g/cm3) 3.218 3.818 3.872 3.8263.878   [3.6]    [3.6]    [3.6] 1.81< nd Refractive index nd 1.667 1.785<1.83 1.793 1.795 1.765 1.745 1.750 Tg, ° C. 622 691 638 690 685 [600] [580] [620] Tx, ° C. 783 822 756 821 815 CTE 20-300 [72] 61.8 [64] 59.867.4 [83]  [75]  [75] CTE 0-100 [66] 57.2 [60] 53.4 61.8 Annealing point(° C.) [670]  686 [625]  687 [690] [600]  [580] [620] Strain point (°C.) [630]  648 [580]  650 [650] Liquidus temperature 1410 1410 (° C.)Young's modulus E 99 118 118 117 114 [95] [110] [100] (GPa) Poisson'sratio n 0.268 0.291 0.289 0.294 0.294    [0.27]    [0.27]    [0.27] (n −1)/d 0.207 0.206 0.211 0.207 0.205   [0.2]    [0.2]    [0.2] Spec,modulus 30.8 30.9 30.5 30.6 29.4 [26]  [30]  [28] alfaE/(1 − nu)   [1.0]1.03   [1.1] 0.99 1.09   [1.1]    [1.1]    [1.0] Tx − Tg 161 131 118 131130 Property Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Density d (g/cm3)   [3.6]    [3.6]    [3.6]    [3.6]    [3.6] Refractive index nd 1.7701.765 1.785 1.800 1.795 Tg, ° C. [600] [600] [600] [560] [600] Tx, ° C.CTE 20-300  [75]  [78]  [78]  [80]   78] CTE 0-100 Annealing point (°C.) [600] [600] [600] [560] [600] Strain point (° C.) Liquidustemperature (° C.) Young's modulus E [110] [110] [110] [110] [120] (GPa)Poisson's ratio n    [0.27]    [0.27]    [0.27]    [0.27]    [0.27] (n −1)/d    [0.2]    [0.2]    [0.2]    [0.2]    [0.2] Spec, modulus  [30] [30]  [30]  [30]  [33] alfaE/(1 − nu)    [1.1]    [1.2]    [1.2]   [1.2]    [1.3] Tx − Tg *Values [in brackets] are estimates calculatedfrom glass compositions. Other values are measured.

The glasses embodiments of Tables 1 and 2 exhibit high refractiveindices, comparatively low densities, rather low coefficients of thermalexpansion and high elastic moduli.

These glasses advantageously have light weight and durability tocracking after loading or mechanical damage. They have high opticaltransmittance in required wavelength ranges, for example, in the visiblerange, or the part of the visible range. Additionally, many of theseglasses are suitable for use in substrates for information recordingmedia because they exhibit a high elastic modulus.

The refractive index determines the thickness of a glass article, e.g.lens, sufficient for its functioning. The most frequently usedcharacteristic of the refractive index is n_(d), i.e. the refractiveindex measured for the wavelength of approximately 589.3 nm,corresponding to the yellow d-line of sodium spectrum. Therefore, thehigher the refractive index, the more compact the optical system of sameperformance can be. For high-index glasses, it is preferable to have thevalue of n_(d) equal to 1.70-1.80 or higher. The glasses describedherein can advantageously provide such high refractive indices.

For many optical systems, such as consumer glasses, smartphone cameras,etc., it is important to have not only small size but light weight aswell. The weight of an article of a given geometrical size and shape isdetermined by density d. For high-index glasses, it is preferable tohave a density value of about 3.5-4.0 g/cm³ or less. The glassesdescribed herein can advantageously have a density value of about3.5-4.0 g/cm³ or less.

A generalized numerical measure of optical performance considering bothhigh refractive index and light weight can be approximately evaluated interms of the ratio (n_(d)−1)/d. This ratio is often mentioned as“refraction”. The higher the ratio (n_(d)−1)/d, the lighter the lens ofsame optical performance is. For high-index glasses it is advantageousto have numerical value of (n_(d)−1)/d of at least about 0.2. At leastsome of the glass embodiments described herein have of at least about0.2. For example, in the glass embodiments described herein 0.2cm³/g≤(N_(d)−1)/d≤0.25 cm³/g.

One more requirement of information recording media is high elasticmodulus, i.e., the ratio of the force exerted upon a substance or bodyto the resultant deformation. High elastic modulus makes a glass articlemore rigid and allows it to avoid large deformations under an externalforce that may take place when recording or reading information. Thereare several numerical characteristics of elasticity. The most commoncharacterization for a material stiffness is the Young's modulus E, i.e.the relationship between stress (force per unit area) and strain(proportional deformation) in an article made of this material. Thegreater the Young's modulus of a material, the less is the deformationof the article. For the substrates of information recording media, it isdesirable to have a Young's modulus of about 100 GPa or higher. Theglass embodiments described herein advantageously have Young's modulusthat is ≥100 GPa.

For optical elements, high stiffness of a material causes a stableoptical image under some external (especially, variable) force, which isimportant for some optical systems. However, high value of the Young'smodulus often conflicts with the technological requirement of lowthermal stress formed in an article during production (see below), whichmakes it desirable to have the stiffness low enough. Accordingly, forhigh-index glasses it is preferable to have the Young's modulus not veryhigh, like 100 GPa or less. For a glass composition that may be used forboth applications, the preferable value of the Young's modulus is around100 GPa.

To characterize glasses that have high Young's modulus and light weight,it is convenient to use the specific modulus E/d, which is the ratio ofthe Young's modulus E to the glass density d. In some exemplaryembodiments, the specific modulus of the glass is 30 GPa·cm³/gram ≤(E/d)40 GPa·cm³/gram. In some exemplary embodiments, the specific modulus ofthe glass is 32 GPa·cm³/gram ≤(E/d) 38 GPa·cm³/gram. In some exemplaryembodiments, the specific modulus of the glass is 30-34 GPa*cm³/gram. Insome exemplary embodiments, the specific modulus of the glass is 32-34GPa*cm³/gram. In some exemplary embodiments, the specific modulus of theglass is 32-40 GPa*cm³/gram. For the glass substrates for informationrecording media, the values of E/d are preferably equal to at leastabout 30 GPa·cm³/gram, for example at least 32 GPa·cm³/gram, or at least30 GPa·cm³/gram, or 34 GPa·cm³/gram.

From a technological viewpoint, it is necessary for optical glasses toavoid excessive stresses that can be formed in a glass article duringproduction. These stresses appear when cooling a glass article afterbeing formed. The value of formed stresses depends on multiple factors,such as size and shape of the article and cooling rate in the sensitivetemperature range (roughly, the interval between the annealing andstrain points of a glass). All other factors being equal, the value ofstress depends on glass composition. This quantity is evaluated as theratio R=E·α/(1−ν), where E represents Young's modulus, α is the meancoefficient of linear thermal expansion and ν is Poisson's ratio; allcharacteristics are measured at temperatures below the glass transitiontemperature T_(g) (the first two of them at room temperature).Accordingly, all other factors being equal, the lower the ratioE·α/(1−ν), the less the values of stress in the glass. There are alsosome other, more complicated indicators that consider structuralrelaxation, but all of them consider, in some way or other, the valuesof thermal expansion coefficient (α) and one of elastic moduli, mostly,the E value. In terms of the ratio R=E·α/(1−ν), optical glassesdescribed herein preferably E·α/(1−ν) ratio of 1.0 MPa·K⁻¹ or less.

Accordingly, the glasses should have rather low coefficients of thermalexpansion (below, we will use the abbreviation α).

For optical elements, as stated above, the value of a contributes tothermal stress that may be formed when cooling an article after hotpressing or other forming procedure, which can be considered in terms ofthe ratio E·α/(1−ν), which preferable values are 1.2 MPa·K⁻¹ or less,for example 1.1 MPa·K⁻¹ or less, or 1.0 MPa·K⁻¹ or less. For example, insome embodiments the ratio E·α/(1−ν) is 0.3 MPa·K⁻¹≤E·α/(1−ν)≤1.0MPa·K⁻¹, or 0.35 MPa·K⁻¹≤E·α/(1−ν)≤1.0 MPa·K⁻¹, or 0.4MPa·K⁻¹≤E·α/(1−ν)≤1.0 MPa·K⁻¹.

Considering that the optimal value of Young's modulus for the describedmulti-purpose glasses is around 100 GPa (see above) and a typical valueof Poisson's ratio for glasses equal to ν≈0.25, the preferable value ofa for these optical glasses is 60-10⁻⁷ K⁻¹ to 80·10⁻⁷ K⁻¹, or 60·10⁻⁷K⁻¹ to 75·10⁻⁷ K⁻¹, more preferably not greater than 72·10⁻⁷ K⁻¹, morepreferably not greater than 70·10⁻⁷ K⁻¹, for example 60·10⁻⁷ K⁻¹ to70·10⁻⁷ K^(−1,) e.g., α≈62·10⁻⁷ K⁻¹, α≈65·10⁻⁷ K⁻¹, α≈67·10⁻⁷ K⁻¹,α≈69·10⁻⁷ K⁻¹.

For glass substrates for information recording media, the value of adetermines the possible changes of the linear size of the substratecaused by temperature changes resulting from recording informationand/or the changes of the outside temperature. The less the value of α,the less the temperature-induced deformation is. The requirements on αof the substrates for information recording media are similar to theprevious case: it is desired to have the value of α be about (60 to70)×10⁻⁷ K⁻¹ or less.

From a technological viewpoint, as stated above, it is important forthese glasses to have the minimum level of the viscosity at liquidustemperature (below, liquidus viscosity) that minimizes susceptibility ofglass melt to crystallization during formation of the articles. Forexample, for optical elements that are formed by hot-pressing or similarmethods, the minimum acceptable liquidus viscosity (depending on thearticle size and equipment) can be several poise and higher. Accordingto the embodiments disclosed herein, the glasses for substrates theinformation recording media advantageously have liquidus viscosity ofabout 100 poise and higher.

Another requirement is the glass transition temperature (T_(g)) thatapproximately characterizes some middle temperature in the range inwhich a glass article becomes mechanically solid when cooling, or startsto soften when heating. Roughly, T_(g) corresponds to the temperature atwhich viscosity is equal to 10¹³ pose; the exact value depends on glasscomposition, heating/cooling rates and other factors. If T_(g) is toolow, a glass article may not be able to sustain heat treatments thatoften take place when manufacturing and/or using glass articles. IfT_(g) is too high, a glass composition may not be compatible withconventional equipment used in production. Therefore, it is preferableto have an intermediate glass transition temperature (e.g. Tg<700° C.).According to at least some embodiments described herein 590° C.≤Tg≤700°C., or 590° C.≤Tg≤650 0° C., or 590° C.≤Tg≤625° C.

Another technological requirement concerns the melting temperature, i.e.the minimum temperature at which the glasses can be melted and refinedfrom the residual gases (coming from the raw materials) during areasonable time. For most industrial glasses, the melting temperature isthe temperature at which a glass melt has a viscosity equal toapproximately 100-300 Pose. The temperature corresponding to theviscosity of 200 Pose (below, we will use the term “200 P temperature”)can be considered as an estimate of the melting temperature. The loweris melting temperature, the lower is the energy consumption for glassmelting. Also, at lower temperatures the process of glass production ismore compatible with conventional refractories, and raising thetemperature may cause corrosion of the refractory elements in thefurnace. Therefore, the lower is the 200 P temperature, the moreadvantageous a given glass composition is (all other factors beingequal) in terms of glass melting. Many of the glass embodimentsdisclosed herein have melting temperature below 1400° C., or below 1375°C., or even at or below 1350° C.

All compositional components, relationships, and ratios described inthis specification are provided in mol %, unless otherwise stated. Allranges disclosed in this specification include any and all ranges andsubranges encompassed by the broadly disclosed ranges whether or notexplicitly stated before or after a range is disclosed.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus, it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A glass composition comprising: (α) Al₂O₃, ZnO,and SiO₂; (b) TiO₂, in the amount of at least 10 mol % and not greaterthan 20 mol %; (c) alkaline metal oxide selected from the groupconsisting of MgO, CaO, SrO, BaO, or any combination thereof, such thatthe molar sum of MgO, CaO, SrO, BaO, and ZnO, in the amount in the glasscomposition is least 20 mol % and not greater than 35 mol %, and suchthat: (i) the amount of BaO is 0 to 10 mol %; (ii) the amount of MgO is0 to 10 mol % (iii) the amount of CaO is 0 to 10 mol %, and the molarsum of CaO and MgO in the glass composition is less than 12.5 mol %; (d)rare earth metal oxides (ΣRE_(m)O_(n)), in the amount of at least 1.5mol % and not greater than 10 mol %; (e) alkali metal oxides (ΣAlk₂O),in the amount of greater than or equal to 0 mol % and less than or equalto 5 mol %; and (f) not greater than 5 mol % of other components; andwherein (g) −5 mol %≤Al₂O₃ (mol %)−1.5 ΣRE_(m)O_(n) (mol %)−ΣAlk₂O (mol%)≤+5 mol %.
 2. The glass composition of claim 1, wherein the glasscomposition comprises greater than or equal to 30 mol % and less than orequal to 45 mol % SiO₂.
 3. The glass composition of claim 1, wherein theglass composition comprises greater than or equal to 15 mol % ZnO. 4.The glass composition of claim 1, wherein the glass compositioncomprises less than or equal to 15 mol % Al₂O₃.
 5. The glass compositionof claim 1, wherein the glass composition has a refractive index n_(d)such as 1.66≤n_(d)≤1.83.
 6. The glass composition of claim 1, whereinthe glass composition has a linear thermal expansion coefficient in therange 20-300° C., α₂₀₋₃₀₀, that is greater than or equal to 60×10⁻⁷ K⁻¹and less than or equal to 70×10⁻⁷ K⁻¹.
 7. The glass composition of claim1, wherein the glass composition has a linear thermal expansioncoefficient in the range 20-100° C., α₂₀₋₁₀₀ of greater than or equal toabout 50×10⁻⁷ K⁻¹ and less than or equal to about 60×10⁻⁷ K⁻¹.
 8. Theglass composition of claim 1, wherein the glass has a Young's modulusthat is greater than or equal to about 95 GPa.
 9. The glass compositionof claim 1, wherein the glass has a Young's modulus that is less than orequal to about 120 GPa.
 10. The glass composition of claim 1, whereinglass density d is: 3.2 g/cm³≤d≤3.9 g/cm³.
 11. The glass composition ofclaim 10, wherein glass density d is less than or equal to about 3.5g/cm³.
 12. The glass composition of claim 1, wherein the glass has aliquidus temperature of less than or equal to about 1410° C.
 13. Theglass composition of claim 1, wherein the glass has a meltingtemperature T_(m) that is less than or equal to 1410° C.
 14. The glasscomposition of claim 1, wherein the glass has a glass transitiontemperature T_(g) that is greater than or equal to than about 600° C.and less than or equal to about 700° C.
 15. The glass composition ofclaim 1, wherein the glass has a crystallization onset temperatureT_(x), such that the difference (T_(x)−T_(g)) is greater than or equalto about 130° C., where T_(g) is the glass transition temperature. 16.The glass composition of claim 1, wherein the glass has a specificmodulus greater than or equal to about 30 GPa*cm³/gram.
 17. The glasscomposition of claim 1, wherein the glass has a ratio α₂₀₋₃₀₀ E/(1−ν)less than or equal to about 1.0 MPa/° C., where α₂₀₋₃₀₀ is a linearthermal expansion coefficient in the range 20-300° C., E is a Young'smodulus, and ν is a Poisson's ratio.
 18. The glass composition of claim1, wherein the glass has a ratio of (n_(d)−1)/d that is greater than orequal to about 0.20 cm³/g.
 19. The glass composition of claim 1, theglass composition comprising: (a) greater than or equal to 30.0 mol %and less than or equal to 35.0 mol % SiO₂; (b) greater than or equal to12.0 mol % and less than or equal to 20.0 mol % TiO₂; (c) greater thanor equal to 10.0 mol % and less than or equal to 15.0 mol % Al₂O₃; (d)greater than or equal to 5.0 mol % and less than or equal to 10.0 mol %of earth metal oxides, (e) greater than or equal to 15.0 mol % and lessthan or equal to 20.0 mol % ZnO; and (f) greater than or equal to 5.0mol % and less than or equal to 15.0 mol % alkaline earth metal oxides(MgO+CaO+SrO+BaO).
 20. The glass composition of claim 19, wherein theglass composition has a refractive index n_(d) such as 1.78≤n_(d)≤1.83.21. The glass composition of claim 19, wherein the glass composition hasa linear thermal expansion coefficient in the range 20-300° C., α₂₀₋₃₀₀,that less than or equal to about 60×10⁻⁷ K⁻¹.
 22. The glass compositionof claim 19, wherein the glass composition has a refractive index n_(d)greater than or equal to 1.75; and a linear thermal expansioncoefficient in the range 20-300° C., α₂₀₋₃₀₀, that less than or equal to65×10⁻⁷ K⁻¹.
 23. A zinc aluminosilicate glass comprising: (a) greaterthan or equal to 10 weight % and less than or equal to 20 weight % TiO₂;(b) greater than or equal to 20 weight % and less than or equal to 35weight % (MgO+CaO+SrO+BaO+ZnO), including a. from 0 to 5 weight % MgO,b. from 0 to 5 weight % CaO, c. from 0 to 10 weight % BaO, d. sum of(CaO+MgO) being less than 10 weight %; (c) greater than or equal to 8weight % and less than or equal to 25 weight % rare earth metal oxides(ΣRE_(m)O_(n)); (d) greater than or equal to 0 weight % and less than orequal to 5 weight % alkali metal oxides (ΣAlk₂O); and (e) not more than5 weight % of other species.
 24. A glass according to claim 23, whereinthe glass composition comprises greater than or equal to 15 weight %SiO₂.
 25. A glass according to claim 23, wherein the glass compositioncomprises greater than or equal to 15 weight % ZnO.
 26. A glassaccording to claim 23, wherein the glass composition comprises less thanor equal to 15 weight % Al₂O₃.
 27. The glass according to claim 23,wherein the glass has the refractive index n_(d) about 1.78-1.83;density less than or equal to about 3.9 g/cm³; glass transitiontemperature T_(g) of 600° C. to 700° C.; specific modulus of about 30-34GPa*cm³/gram; and liquidus temperature of less than or equal to 1410°C., and crystallization onset temperature T_(x) greater than or equal to(T_(g)+130) ° C.
 28. The glass composition according to claim 1,comprising at least 30.0 mol % SiO₂, said glass being free of iron,lead, antimony ant tantalum oxides, said glass having a refractive indexn_(d) greater than or equal to 1.75 and a linear thermal expansioncoefficient in the range 20-300° C., α₂₀₋₃₀₀, that is less than or equalto about 65×10⁻⁷ K⁻¹.
 29. The glass composition according to claim 1,comprising at least 30 to 35 mol % SiO₂, 10 to 15 mol % Al₂O₃, f15 to 20mol % TiO₂, 15 to 20 mol % ZnO, and 5 to 10 mol % La₂O₃.
 30. A glassarticle comprising a zinc aluminosilicate glass, the glass comprising,on mole percent basis: a) greater than or equal to 10 mol % and lessthan or equal to 20 mol % TiO₂; b) greater than or equal to 20 mol % andless than or equal to 35 mol % (MgO+CaO+SrO+BaO+ZnO), including from 0to 10 mol % MgO, from 0 to 10 mol % CaO, from 0 to 10 mol % BaO, sum of(CaO+MgO) being less than 12.5 mol %; c) greater than or equal to 1.5mol % and less than or equal to 10 mol % rare earth metal oxides(ΣRE_(m)O_(n)); d) greater than or equal to 0 mol % and less than orequal to 5 mol % alkali metal oxides (ΣAlk₂O); and e) not more than 5mol % of other compatible species, f) wherein the following issatisfied: −5 mol %≤(Al₂O₃[mol %]−1.5 ΣRE_(m)O_(n) [mol %]−ΣAlk₂O [mol%])≤+5 mol %.